Real name David Oh. Zero is the founder and former commander of the Special Forces unit FOX. A British national, Zero is a veteran of the SAS who served alongside The Boss. He used his SAS experience to propose an entirely new kind of Special Forces unit, one whose intelligence capabilities would make it perfect for top-secret Cold War missions. He pitched his idea for FOX to the CIA and to prove its utility, he agreed to take on the Virtuous Mission. Zero supported Naked Snake on his mission over the radio using "Major Tom" as his codename in a nod to the movie "The Great Escape".

The Virtuous Mission was launched on August 24, 1964. Para-Medic and The Boss were deployed as a rear support team, while Naked Snake was sent in as the field agent to sneak into Tselinoyarsk, USSR. But The Boss's defection resulted in the failure of the mission's objective, to bring Sokolov back to the US. Colonel Volgin of the GRU destroyed the secret Soviet design bureau OKB-754 with a Davy Crockett missile and framed the United States for the deed, sparking an international crisis.

On August 30, 1964, to prove its innocence, the US decided to assassinate the traitorous Boss. With the survival of FOX on the line, Zero launched Operation Snake Eater. Once again, Naked Snake was chosen as the field agent, with Para-Medic and Sigint providing rear support. The mission was a success: the Shagohod was destroyed, The Boss was eliminated, and America recovered a roll of microfilm revealing the Philosophers' Legacy. Upon returning home, Naked Snake earned the title Big Boss.

Zero's stature rose and his influence grew inside the halls of Langley. In 1970, he conspired with Gene to grab the remainder of the Philosophers' Legacy by staging the uprising on the San Hieronymo Peninsula and stealing Metal Gear. But the plot failed due to Gene's intransigence and the exploits of Naked Snake. Revolver Ocelot succeeded in assassinating the director of the CIA, but was unable to obtain the entire Philosophers' Legacy.